U.S. officials say that investigators have recovered personal and work-related emails that former secretary of state Hillary Clinton believed had been deleted from the private server she used while serving as the country's top diplomat from 2009 to 2013.

The Federal Bureau of Investigation has been probing how classified national security material ended up on Clinton's email server.

Clinton, now the leading candidate for the 2016 Democratic presidential nomination, has said she used the server as a convenience so she did not have to maintain two email accounts, one for State Department business and one for personal matters.

It was not clear how many of the 60,000 emails on the server have been recovered. Clinton has said the server was wiped clean, but one investigator told The New York Times that it had not been hard to recover the emails.

Clinton says that 30,000 of them she deemed personal had been discarded and copies of the remainder turned over to the State Department. Officials there are sorting through them and have found dozens of classified documents, although Clinton says she is confident that none of the emails was marked as confidential when she sent or received it.

The State Department has released several batches of the official emails, and is planning to disclose even more of them in the coming weeks. The emails recovered from the server could also eventually be made public.

Republican political opponents of Clinton in Congress are combing through the emails to investigate her role during the 2012 attack on a U.S. diplomatic outpost in Benghazi, Libya, that killed four Americans, including the U.S. ambassador there.

With the ongoing controversy over her emails, surveys about the presidential race show her political standing has waned, with increasing numbers of voters questioning her trustworthiness. But the surveys also show that Democratic voters still favor Clinton for the party's presidential nomination next year.